One of the ever-present problems of sensitive electronic instruments such as ECG machines that are used to provide signals indicative of the electrical activity of the heart is the presence of hum in the output signal. Whereas an instrument can often be designed so as to be fairly free from hum if it is operating in a relatively weak field environment, this is not always the case, the other equipment is usually present. Generally, the hum is an AC wave of the power line frequency, but it can have many harmonics picked up from nearby equipment such as those produced by the full wave rectifier of X-ray machines. In fact, the harmonics may have greater amplitude than the power line frequency.
One way of reducing hum is to pass the signal through an electronic notch filter having tuned circuits that provide maximum attenuation for a single predetermined frequency. If the hum contains the power line frequency as well as harmonics thereof, a number of notch filters must be used, one for each frequency to be attenuated. With most designs, the Q of these filters is not nearly as high as desired so that signal frequencies that lie considerably above and below that to which the filter is tuned are also attenuated. Furthermore, if high Q's are attained, variation in the values of the circuit components of the filter as well as variations in the power line frequency can cause the maximum attenuation of the filter to occur at a frequency different from that of the hum.